I don't know about Diety and an extra settler, but I know that a worker was removed at each level that the AI got one, which makes a big difference in how fast they expand. Consider that I'm usually willing to spend my entire food/hammer output for 15 turns to get that first worker, it's a really big deal. Also happy caps are raised at high levels, and AI maintainence/upgrade costs have been flattened. All of these "easier factors" only apply at high levels, but together they definitely make those levels easier. I can't speak for mid-levels, but I'm guessing it's more of a wash -- the AI techs a bit slower but fights better.
It's also definitely true that the balance has shifted to favor peaceful expansion as long as there's reasonably good land, rather than early war. So depending on your playstyle that could either help or hurt.
peace,
lilnev